I think the tech support person might have been confused - noise gates don't usually remove a specific type of noise from your recording, they just cut out the signal when it drops below a certain level. So, when you're talking, you'll still hear the hum in the background, but when you stop talking, the hum (and all background noise) can be reduced to zero.
To set the latter up, you just need to play with the threshold setting on the noise gate, and tweak that to strike a balance - too far in one direction, and it'll cut off your speech, too far in the other direction, and it'll let too much of the noise between sentences through. If the gate offers an "attack" value, set that to the minimum, and if it offers a "release" value, something like 200ms is probably fine.
If you want to be able to remove the hum from everything, whether you're speaking or not, the best tool for that - assuming you can't fix it with the microphone - is a parametric equaliser. Use a spectrum analyzer (I forget if Sonar contains one, but SPAN is free if not -
http://www.voxengo.com/product/span/) on the channel to see what frequency the humming is at, then use an equaliser to reduce that frequency.